The solution studies of binary complex of Rifampicin with Ce(III), Dy(III) ions have been performed. The protonation constant of Rifampicin and the stability constant of complexes with Ce(III), Dy(III) ions at 303K were investigated. The ionic strength 0.01M is constantly maintained by adding NaClO₄. The pK and logK values of the complexes were determined in 10% to 50% Dioxane Water medium by the pH metric method. The pK and logK values of the complexes increased as dioxane content increased.